Özet:

The atomic clocks with cheap internal oscillators are usually used in GNSS (Global Navigation Satellite Systems) receivers rather than highly stable and accurate atomic clocks used in GNSS satellites. In order to be synchronized with GPS time, the receivers employing the atomic clocks equipped with crystal oscillators get periodical corrections to the GNSS observations when internal clock offset exceeds a threshold. These corrections so-called clock jumps are typically as an integer number of milliseconds and their amount is the same for the whole received signals, unlike the cycle slips. Since these clock jumps influence the time tag, pseudorange and carrier phase measurements produced by GNSS receivers, their types and amounts change from one receiver to another. Especially, when the clock jumps have different effects on code and phase measurements, they cause clock discontinuity and if they are not detected appropriately, they negatively affect the positioning performance. Although the impact of clock jumps is mostly ignored in differential and relative positioning techniques as they are eliminated by using double-differences, it is required to detect and repair clock jumps properly in absolute positioning techniques, such as Precise Point Positioning (PPP). In this study, the types of clock jumps depending on their effects on the measurements are introduced firstly. Then, the algorithm developed to detect and repair the clock jumps is explained in details. The experimental tests which include real GNSS data indicate that the algorithm can successfully be used for detecting clock jumps. Finally, an additional test, which is conducted to investigate how the positioning performance is influenced when the clock jumps are not appropriately detected, and its results are provided as a part of this study. The results present that if the clock jumps are not detected properly, they affect the positioning performance of PPP negatively in terms of positioning accuracy and convergence time.

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